The present invention relates to hydrophones employed in seismic exploration. More particularly, the invention relates to an improved hydrophone circuit that provides the frequency response characteristics of an accelerometer.
Due to the increasing difficulty and cost of finding petroleum resources in the world today, exploration techniques are becoming more and more technologically sophisticated. For example, many have found crystal hydrophones to be useful in petroleum exploration. Basically, hydrophones are used to measure seismic waves created by a source such as an air gun or a dynamite charge, to obtain detailed information about various sub-surface strata of earth.
As shown in FIG. 1A, a typical crystal hydrophone 100 includes a diaphragm 102, a crystal 104, and a housing 106 that is typically filled with a gas 107. The diaphragm 102, which has front and rear sides 102a, 102b, is made from a material such as Kovar or a Beryllium Copper compound, and is electrically connected to the crystal by a conductive epoxy 108. The crystal 104 is typically made from a material such as Lead Zirconium Titanate, and is silver-plated on its top 104a and bottom 104b to achieve better conductivity. The crystal 104 is initially polarized by applying a high-voltage electrical charge to the crystal 104. When the polarized crystal 104 experiences pressure resulting from a physical input such as sound, fluid pressure, or another type of pressure, it produces a voltage representative of the pressure experienced. The crystal 104 is electrically connected to electrical output leads 110, 112. To protect the crystal 104 from contaminants, and to maintain the crystal 104 in atmospheric pressure, the crystal 104 and the rear side 102b of the diaphragm 102 are sealed within the gas-filled housing 106. The housing 106 protects the crystal 104 and diaphragm 102, and facilitates mounting of the hydrophone 100.
The diaphragm 102 functions to vibrate in response to physical pressures it experiences. The physical deflection of the diaphragm 102 is transferred by the epoxy 108 to the crystal 104, deforming the electron structure of the crystal 104 and causing an electrical potential to be provided across the leads 110, 112.
Another apparatus that is also useful in petroleum exploration is the accelerometer. Accelerometers are commonly used to measure the motion of the earth""s surface in response to seismic waves created by a seismic source, to obtain detailed information about various sub-surface strata in the earth.
As mentioned above, hydrophones and accelerometers are often used in petroleum exploration in conjunction with seismic equipment. In one example of such an application (FIG. 1B), a cable 150 including one or more hydrophones and one or more accelerometers is placed on the sea floor 154. Such a cable may be made up of cylindrical units 152, where each unit 152 includes a geophone and an accelerometer.
Seismic waves are produced by a seismic source 156 that is towed behind a ship 158; the seismic source 156 may comprise an air gun, a dynamite charge, or the like. The seismic source 156 produces a large explosion, creating seismic waves 160. The seismic waves 160 travel through water 162 and various layers of earth 164, and are reflected back to the cable 150 as upgoing incident waves 161. Each unit 152 detects and measures the incident waves 161 and creates a real-time record of the results. This record is typically stored in a recorder (not 20 shown) that is linked to or contained within the cable 150. Records of this nature help geologists determine the makeup of the earth 164.
One problem with this arrangement, however, is surface ghost signals 166. Surface ghost signals 166 are produced by incident waves 161 that are reflected from the water""s surface 168. At the wavelengths typically used for seismic signals, the surface 168 provides an effective mirror to reflect incident waves 161 and create downgoing surface ghost signals 166. Surface ghost signals 166 contain no additional information regarding the composition of the earth 164 or the possible petroleum deposits therein, and they interfere with the proper receipt and interpretation of the incident waves 161. Accordingly, it is desirable to eliminate the errors introduced by the surface ghost signals 166.
A hydrophone-accelerometer combination, in theory, is naturally suited to eliminate surface ghost signals. Generally, hydrophones detect pressure omnidirectionally, and accelerometers detect force or acceleration, which is directional. Due to the relative strengths of the incident waves 161 and the surface ghost signals 166 at different depths, a hydrophone""s output and an accelerometer""s output will both vary with depth. For a seismic wave 161 of a given magnitude and frequency, a hydrophone""s output will vary with depth sinusoidally (curve 180, FIG. 1C). Likewise, for the given seismic wave 161, an accelerometer""s output will vary sinusoidally with depth (curve 182, FIG. 1C). The hydrophone and accelerometer outputs may be scaled by external circuitry or by a mathematical algorithm in a computer, so that their peak values have the same amplitude; for example, in FIG. 1C, the hydrophone and accelerometer outputs are scaled to a maximum peak amplitude of 1 and a minimum peak amplitude of xe2x88x921. After such scaling, the sum of the hydrophone and accelerometer outputs will always be 1, irrespective of the depth at which the hydrophone and accelerometer are both located (curve 184, FIG. 1C). Therefore, in theory, a hydrophone output and an accelerometer output may be combined to effectively eliminate the influence of surface ghost signals 166.
One problem in applying this theory is that the frequency responses of hydrophones and accelerometers differ. Therefore, the hydrophone and accelerometer outputs will only complement each other as shown in FIG. 1C when the seismic wave 160 has a certain frequency. As a result, if the frequency of the seismic wave 160 were to change, the combined hydrophone-accelerometer output 184 would no longer be constant.
The difference between frequency responses of hydrophones and accelerometers will now be explained with reference to FIGS. 2-4B. When an electronic amplifier 200 (FIG. 2) is utilized to amplify the output of a typical hydrophone 202, the frequency response of the hydrophone 202 (FIGS. 3A, 3B) resembles that of a single-pole high pass filter, since it exhibits a single pole and a 6 dB/octave slope at frequencies less than its natural frequency (fn). The amplifier 200 may comprise an operational amplifier. The hydrophone may be modeled as a voltage source 202a and a capacitor 202b and resistor 202c in series; the capacitor 202b and the resistors 202c and 204 provide the single pole, and hence the 6 dB/octave slope. The natural frequency of the hydrophone 202 depends upon the value of the internal resistance 204 (Ri) of the amplifier 200, the resistance (RH) of the resistor 202c, and the capacitance (CH) of the capacitor 202b; this relationship is shown in equation 1.0, below.       f    n    =            1              2        ⁢                  π          ⁡                      (                                          R                H                            +                              R                I                                      )                          ⁢        C              ⁢          (      Hz      )      
For typical hydrophones, the natural frequency ranges from about 2 to 3 Hz.
In contrast to the hydrophone 202, as illustrated in FIGS. 4A and 4B, the frequency response of a typical force-balance accelerometer, such as that disclosed in U.S. Pat. No. 5,852,242, issued on Dec. 22, 1998, the disclosure of which is incorporated herein by reference, resembles an electrical circuit having a differentiating element in combination with a pair of simple lag elements. The resulting frequency response exhibits a 6 dB/octave slope at frequencies less than a first cut-off frequency (Fc1), a substantially flat response between the first cut-off frequency (Fc1) and a second cut-off frequency (F2), and a xe2x88x926 dB/octave slope at frequencies greater than the second cut-off frequency (Fc2). For typical force-balance accelerometers, the first cut-off frequency (Fc1) ranges from about 1 to 10 Hz, and the second cut-off frequency (Fc2) ranges from about 1K to 100K Hz.
For the reasons explained above, hydrophones and accelerometers have different frequency response characteristics. Accordingly, hydrophones and accelerometers are not naturally suited to eliminate ghost signals 166 across the whole spectrum of desired frequency. To use a hydrophone with an accelerometer advantageously, the frequency response of the hydrophone must match the frequency response of the accelerometer.
The present invention is directed to overcoming one or more of the limitations of conventional hydrophones.
According to one aspect of the present invention, a hydrophone assembly is provided that includes a hydrophone and a hydrophone filter coupled to the hydrophone. The frequency response of the hydrophone assembly matches the frequency response of an accelerometer.
According to another aspect of the present invention, an apparatus for measuring seismic waves is provided that includes an accelerometer and a hydrophone assembly. The hydrophone assembly includes a hydrophone and a hydrophone filter coupled to the hydrophone. The frequency response of the hydrophone assembly matches the frequency response of an accelerometer.
According to another aspect of the present invention, a marine seismic acquisition system is provided that includes a seismic source for generating seismic energy, a hydrophone for detecting seismic energy, a hydrophone filter coupled to the hydrophone, an accelerometer for detecting seismic energy, a seismic recorder coupled to the accelerometer and the hydrophone filter, and a controller coupled to the seismic source and seismic recorder for controlling and monitoring the operation of the seismic source and seismic recorder. The frequency response of the combination of the hydrophone and hydrophone filter matches the frequency response of the accelerometer.
According to another aspect of the present invention, a method of providing a hydrophone assembly having a frequency response that matches that of an accelerometer is provided that includes filtering the output of the hydrophone with a circuit that provides a differentiator and a pair of simple lags.
According to another aspect of the present invention, a method of measuring seismic energy using a hydrophone assembly and an accelerometer is provided that includes placing the hydrophone assembly and accelerometer in a body of water, generating seismic energy in the body of water, measuring the seismic energy using the hydrophone assembly and the accelerometer, scaling the output of either the accelerometer or hydrophone assembly, and generating an output signal substantially free from surface ghost signals by summing scaled output with the non-scaled output. The frequency response of the hydrophone assembly matches the frequency response of the accelerometer.